Differential velocity sensor

ABSTRACT

A method, system, and apparatus for determining the location of a tool traveling down a wellbore by measuring a first borehole magnetic anomaly with respect to time at two known locations on a tool, comparing the time difference between the two measurements, then calculating the velocity of the tool based on the comparison and then further calculating the distance traveled by the tool in the wellbore based on the velocity calculation.

RELATED APPLICATIONS

This application is a U.S. continuation of U.S. Non-provisional patentapplication Ser. No. 16/079,395 filed Aug. 23, 2019, which is a 371 ofInternational Application No. PCT/US17/19190 filed Feb. 23, 2017, whichclaims priority to U.S. Provisional Application No. 62/298,782, filed onFeb. 23, 2016.

BACKGROUND OF THE INVENTION

Generally, when completing a subterranean well for the production offluids, minerals, or gases from underground reservoirs, several types oftubulars are placed downhole as part of the drilling, exploration, andcompletions process. These tubulars can include casing, tubing, pipes,liners, and devices conveyed downhole by tubulars of various types. Eachwell is unique, so combinations of different tubulars may be loweredinto a well for a multitude of purposes.

A subsurface or subterranean well transits one or more formations. Theformation is a body of rock or strata that contains one or morecompositions. The formation is treated as a continuous body. Within theformation hydrocarbon deposits may exist. Typically a wellbore will bedrilled from a surface location, placing a hole into a formation ofinterest. Completion equipment will be put into place, including casing,tubing, and other downhole equipment as needed. Perforating the casingand the formation with a perforating gun is a well known method in theart for accessing hydrocarbon deposits within a formation from awellbore.

Explosively perforating the formation using a shaped charge is a widelyknown method for completing an oil well. A shaped charge is a term ofart for a device that when detonated generates a focused explosiveoutput. This is achieved in part by the geometry of the explosive inconjunction with an adjacent liner. Generally, a shaped charge includesa metal case that contains an explosive material with a concave shape,which has a thin metal liner on the inner surface. Many materials areused for the liner; some of the more common metals include brass,copper, tungsten, and lead. When the explosive detonates the liner metalis compressed into a super-heated, super pressurized jet that canpenetrate metal, concrete, and rock.

A perforating gun has a gun body. The gun body typically is composed ofmetal and is cylindrical in shape. Within a typical gun tube is a chargeholder or carrier tube, which is a tube that is designed to hold theactual shaped charges. The charge holder will contain cutouts calledcharge holes where the shaped charges will be placed.

A shaped charge is typically detonated by a booster or igniter. Shapedcharges may be detonated by electrical igniters, pressure activatedigniters, or detonating cord. One way to ignite several shaped chargesis to connect a common detonating cord that is placed proximate to theigniter of each shaped charge. The detonating cord is comprised ofmaterial that explodes upon ignition. The energy of the explodingdetonating cord can ignite shaped charges that are properly placedproximate to the detonating cord. Often a series of shaped charges maybe daisy chained together using detonating cord.

Another type of explosive used in completions is a jet cutter. This isan explosive that creates a radial explosion. It can be used to severtubulars, including downhole casing.

A firing head is used to detonate the detonating cord in the perforatinggun. The firing head may be activated by an electrical signal.Electricity may be provided by a wireline that ties into the cableheadat the top of a tool string. The electrical signal may have to travelthrough several components, subs, and tools before it gets to the firinghead. A reliable electrical connector is needed to ensure the electricalsignal can easily pass from one component to the next as it moves downthe tool string. The electrical signal is typically grounded against thetool string casing. As a result, the electrical connections must beinsulated from tool components that are in electrical contact with thetool string casing.

SUMMARY OF EXAMPLE EMBODIMENTS

An example embodiment may include an apparatus for use downholeincluding a top housing with a first end, a second end, an axis, abottom housing with a first end located proximate to the second end ofthe top housing, and a second end, wherein the bottom housing is coaxialwith the axis, a first magnetic anomaly sensor located within the tophousing, a second magnetic anomaly sensor located within the bottomhousing and located a fixed axial distance from the first magneticanomaly sensor, and a processor located within the top housing,operatively connected to the first magnetic anomaly sensor and thesecond magnetic anomaly sensor, in which the processor calculates thevelocity of the apparatus based on comparing measurements taken from thefirst magnetic anomaly sensor and the second magnetic anomaly sensor.

A variation of the example embodiment may include having a plurality ofelectromagnetic coils disposed within the first magnetic anomaly sensor.It may have a first electromagnetic coil disposed within the firstmagnetic anomaly sensor adapted to generate an electromagnetic field.There may be a second electromagnetic coil disposed within the firstmagnetic anomaly sensor adapted to generate an electromagnetic field.There may be a third electromagnetic coil disposed within the firstmagnetic anomaly sensor adapted to detect an electromagnetic field.There may be a fourth electromagnetic coil disposed within the firstmagnetic anomaly sensor adapted to detect an electromagnetic field.There may be a fifth electromagnetic coil disposed within the firstmagnetic anomaly sensor adapted to detect an electromagnetic field.There may be a sixth electromagnetic coil disposed within the firstmagnetic anomaly sensor adapted to detect an electromagnetic field.

A variation of the example embodiment may include a first sub coupled tothe first end of the top housing. It may include a second sub coupled tothe second end of the top housing and coupled to the first end of thebottom housing. It may include a third sub coupled to the second end ofthe bottom housing. The first centralizer may have a hollow cylindricalshape. The second centralizer may have a substantially hollowcylindrical shape.

A variation of the example embodiment may include a cylindrical corelocated coaxial with the axis and passing through the first, second,third, fourth, fifth, and sixth electromagnets. There may be a pluralityof electromagnetic coils disposed within the second magnetic anomalysensor. There may be a seventh electromagnetic coil disposed within thesecond magnetic anomaly sensor adapted to generate an electromagneticfield. It may have an eighth electromagnetic coil disposed within thesecond magnetic anomaly sensor adapted to generate an electromagneticfield. It may have a ninth electromagnetic coil disposed within thesecond magnetic anomaly sensor adapted to detect an electromagneticfield. It may have a tenth electromagnetic coil disposed within thesecond magnetic anomaly sensor adapted to detect an electromagneticfield. It may have an eleventh electromagnetic coil disposed within thesecond magnetic anomaly sensor adapted to detect an electromagneticfield. It may have a twelfth electromagnetic coil disposed within thesecond magnetic anomaly sensor adapted to detect an electromagneticfield.

A variation of the example embodiment may include a cylindrical corelocated coaxial with the axis and passing through the first, second,third, fourth, fifth, and sixth electromagnets. The processor includes adata logger. The processor may include a plurality of processors. Theprocessor may compute the velocity by comparing measurements taken fromthe first magnetic anomaly sensor and the second magnetic anomalysensor. It may include a first centralizer surrounding a portion of thefirst end of the top housing. It may include a second centralizersurrounding a portion of the second end of the top housing and a portionof the second end of the bottom housing. The top housing may be composedof a frangible material. The top housing may be composed of a ceramicmaterial. The top housing may be composed of steel. The bottom housingmay be composed of a frangible material. The bottom housing may becomposed of a ceramic material. The bottom housing may be composed ofsteel. The processor may calculate distance traveled by integrating thecalculated velocity with respect to time. The processor may calculatethe distance traveled using a summation of the calculated velocity withrespect to time. The processor may calculate the distance traveled byaveraging the calculated velocity over a plurality of measurements andmultiplying by time. The processor may calculate the distance traveledusing a piecewise summation with respect to time.

An example embodiment may include an apparatus for use downholeincluding a cylindrical housing with a first end, a second end, an axis,a first magnetic anomaly sensor located within the cylindrical housing,a second magnetic anomaly sensor located within the cylindrical housingand located a fixed axial distance from the first magnetic anomalysensor, and a processor located within the cylindrical housing,operatively connected to the first magnetic anomaly sensor and thesecond magnetic anomaly sensor, in which the processor compares themeasurements of the first magnetic anomaly sensor, the second magneticanomaly sensor, the time differential of those measurements, and withthe fixed axial distance between the two sensors, calculates theinstantaneous velocity of the tool.

A variation of the example may include having a plurality of processors.It may have stored log data of the wellbore and compare that to the twomeasurements to fine tune the velocity calculation. The first magneticanomaly sensor may include a plurality of electromagnetic coils orientedabout the axis. The second magnetic anomaly sensor may include aplurality of electromagnetic coils wrapped oriented about the axis. Thecylindrical housing may be composed of a frangible material. Thecylindrical housing may be composed of a ceramic material. Thecylindrical housing may be composed of steel. The processor maycalculate the distance traveled by the tool based on the calculatedinstantaneous velocity. The processor may calculate the distancetraveled by the tool by integrating the calculated velocity with respectto time. The processor may calculate the distance traveled by the toolusing summation of the calculated velocity with respect to time. Theprocessor may calculate the distance traveled by the tool by averagingthe calculated velocity over a plurality of measurements and multiplyingby time. The processor may calculate the distance traveled by the toolusing a piecewise summation with respect to time.

An example embodiment may include a method for determining the locationof a tool in a wellbore including measuring a first borehole magneticanomaly with respect to time at a first location on a tool, measuringthe first borehole magnetic anomaly with respect to time at a secondlocation on a tool a predetermined distance from first location,comparing the time difference between the first magnetic anomaly at thefirst location with the first magnetic anomaly at the second location,calculating the velocity of the tool based on the comparison of the timedifference of the first magnetic anomaly at the first location with thefirst magnetic anomaly at the second location, the time, and thedistance between the first location and the second location, andcalculating the distance traveled by the tool based on the velocitycalculation.

A variation of the example embodiment may include executing apreprogrammed function when the tool travels a predetermined distance.It may include comparing the measured first magnetic anomaly at thefirst location with log data. It may correct the measured first magneticanomaly at the first location with log data. It may compare the measuredfirst magnetic anomaly at the second location with log data. It maycorrect the measured first magnetic anomaly at the second location withlog data. It may measure time to determine the time differential betweenthe measurement at the first location and the measurement at the secondlocation. It may generate a first electromagnetic field. It may generatea second electromagnetic field. The calculation of the distance mayinclude integrating the calculated velocity with respect to time.Calculating the distance may include summation of the calculatedvelocity with respect to time. Calculating the distance may includeaveraging the calculated velocity over a plurality of measurements andmultiplying by time. Calculating the distance may include a piecewisesummation with respect to time.

An example embodiment may be a system for use downhole including aplugging tool having a cylindrical housing, a first end, a distal end,an axis, and a packer, an autonomous tool with a first end, a secondend, located coaxial with the axis, wherein the second end of theautonomous tool is coupled to the first end of the plugging tool, theautonomous tool further comprising, a top housing with a first end, asecond end, located coaxial with the axis, a bottom housing with a firstend located proximate to the second end of the top housing, and a secondend, wherein the bottom housing is coaxial with the axis, a firstmagnetic anomaly sensor located within the first housing, a secondmagnetic anomaly sensor located with the second housing, and a processorlocated within the top housing, operatively connected to the firstmagnetic anomaly sensor and the second magnetic anomaly sensor, whereinthe processor compares data from the first magnetic anomaly sensor andthe second magnetic anomaly sensor to determine the velocity of theautonomous tool and then calculating the distance the autonomous toolhas traveled downhole using the calculated velocity.

A variation of the example embodiment may have a plurality ofelectromagnetic coils disposed within the first magnetic anomaly sensor.A first electromagnetic coil may be disposed within the first magneticanomaly sensor adapted to generate an electromagnetic field. A secondelectromagnetic coil may be disposed within the first magnetic anomalysensor adapted to generate an electromagnetic field. A thirdelectromagnetic coil may be disposed within the first magnetic anomalysensor adapted to detect an electromagnetic field. A fourthelectromagnetic coil may be disposed within the first magnetic anomalysensor adapted to detect an electromagnetic field. A fifthelectromagnetic coil may be disposed within the first magnetic anomalysensor adapted to detect an electromagnetic field. A sixthelectromagnetic coil may be disposed within the first magnetic anomalysensor adapted to detect an electromagnetic field.

Further variations of the example embodiment may include a first subbeing coupled to the first end of the top housing. A second sub may becoupled to the second end of the top housing and coupled to the firstend of the bottom housing. A third sub may be coupled to the second endof the bottom housing. The first centralizer may have a hollowcylindrical shape. The second centralizer may have a substantiallyhollow cylindrical shape. A cylindrical core may be located coaxial withthe axis and passing through the first, second, third, fourth, fifth,and sixth electromagnets. It may include a plurality of electromagneticcoils disposed within the second magnetic anomaly sensor. A seventhelectromagnetic coil may be disposed within the second magnetic anomalysensor adapted to generate an electromagnetic field. An eighthelectromagnetic coil may be disposed within the second magnetic anomalysensor adapted to generate an electromagnetic field. A ninthelectromagnetic coil may be disposed within the second magnetic anomalysensor adapted to detect an electromagnetic field. A tenthelectromagnetic coil may be disposed within the second magnetic anomalysensor adapted to detect an electromagnetic field. An eleventhelectromagnetic coil may be disposed within the second magnetic anomalysensor adapted to detect an electromagnetic field. A twelfthelectromagnetic coil may be disposed within the second magnetic anomalysensor adapted to detect an electromagnetic field.

Further variations of the example embodiment may include a cylindricalcore located coaxial with the axis and passing through the first,second, third, fourth, fifth, and sixth electromagnets. The processormay include a data logger. The processor may include a plurality ofprocessors. The processor may compute the velocity by comparingmeasurements taken from the first magnetic anomaly sensor and the secondmagnetic anomaly sensor. A first centralizer may surround a portion ofthe first end of the top housing. A second centralizer may surround aportion of the second end of the top housing and a portion of the secondend of the bottom housing. The top housing may be composed of afrangible material. The top housing may be composed of a ceramicmaterial. The top housing may be composed of steel. The bottom housingmay be composed of a frangible material. The bottom housing may becomposed of a ceramic material. The bottom housing may be composed ofsteel. The packer may be composed of metal. The packer may be composedof a hard rubber. A braking assembly may be coupled to the first end ofthe top housing. A jet cutter may be coupled to the braking assembly. Ajet cutter may be coupled to autonomous tool.

Further variations of the disclosed embodiments may include theprocessor calculating the distance traveled by the tool based on thecalculated instantaneous velocity. The processor may calculate thedistance traveled by the tool by integrating the calculated velocitywith respect to time. The processor may calculate the distance traveledby the tool using summation of the calculated velocity with respect totime. The processor may calculate the distance traveled by the tool byaveraging the calculated velocity over a plurality of measurements andmultiplying by time. The processor may calculate the distance traveledby the tool using a piecewise summation with respect to time.

An example embodiment may include a method for locating a downhole toolincluding inserting an autonomous tool into a borehole, moving theautonomous tool down the borehole, programming the autonomous tool toexecute a command at a predetermined location within the borehole,detecting a set of borehole magnetic anomalies at a first location onthe autonomous tool, detecting the set of borehole magnetic anomalies ata second location on the autonomous tool, comparing the detection at thefirst location with the detection at the second location, calculatingthe velocity of the autonomous tool based on the comparison the set ofborehole magnetic anomalies measured at the first location and secondlocation, calculating the position of the tool based on the calculatedvelocity, and automatically executing a command when the autonomous toolreaches a predetermined location.

A variation of the embodiment may include the autonomous tool generatingan electromagnetic field at a first location in the autonomous tool. Theautonomous tool may generate an electromagnetic field at a secondlocation in the autonomous tool. It may detect casing collars based onthe detected borehole magnetic anomalies. It may execute a command tofire a perforating gun. It may execute a command to deploy a brakeassembly. It may execute a command to fire a pipe severing tool. It mayexecuted a command to expand a plug within the borehole. It may move theautonomous tool by dropping it down a wellbore. Moving the autonomoustool may include pumping it down a wellbore. It may calculate theposition by integrating the calculated velocity with respect to time. Itmay calculate the position by a summation of the calculated velocitywith respect to time. It may calculate the position by averaging thecalculated velocity over a plurality of measurements and multiplying bytime. It may calculate the position of the tool using a piecewisesummation with respect to time.

BRIEF DESCRIPTION OF THE DRAWINGS

For a thorough understanding of the present invention, reference is madeto the following detailed description of the preferred embodiments,taken in conjunction with the accompanying drawings in which referencenumbers designate like or similar elements throughout the severalfigures of the drawing. Briefly:

FIG. 1 shows a side view of an autonomous downhole tool.

FIG. 2 an assembly view of an autonomous downhole tool.

FIG. 3 shows a close up of a magnetic anomaly sensor used in anautonomous downhole tool.

FIG. 4 shows an autonomous tool combined with a perforating gun.

FIG. 5 shows an autonomous tool combined with a perforating gun andwireline cablehead.

FIG. 6 shows an autonomous tool with two magnetic anomaly sensors spaceda fixed axial distance apart.

FIG. 7 shows an autonomous tool combined with a perforating gun in adifferent configuration.

FIG. 8 . Shows an autonomous tool with two magnetic anomaly sensorscombined with a setting tool and a jet cutter.

DETAILED DESCRIPTION OF EXAMPLES OF THE INVENTION

In the following description, certain terms have been used for brevity,clarity, and examples. No unnecessary limitations are to be impliedtherefrom and such terms are used for descriptive purposes only and areintended to be broadly construed. The different apparatus, systems andmethod steps described herein may be used alone or in combination withother apparatus, systems and method steps. It is to be expected thatvarious equivalents, alternatives, and modifications are possible withinthe scope of the appended claims.

An example embodiment is shown in FIG. 1 depicting an autonomous tool10. The autonomous tool 10 has a first housing 11, a second housing 12,a first centralizer 13, and a second centralizer 14. The first housing11 has a first end 37 and a second end 26. The first end 15 of theautonomous tool 10 ends with a top sub 16. The top sub 16 has a hollowcenter 17 with a wire port plug 18. The first housing 11 is hollow andcontains a chassis 19 for holding a battery 20, data logger 21, and aprinted circuit board (PCB) support 22. The PCB support contains a PCB23.

Still referring to FIG. 1 a central sub 24 is coupled to the second end26 of the first housing 11 and the first end 27 of the second housing12. The second centralizer 14 is positioned over the first housing 11,second housing 12, and central sub 24. The central sub 24 is hollow toallow for electronic connections to connect components in the firsthousing 11 with components in the second housing 12. The second housing12 contains a coil assembly 25. The second end 28 of the second housing12 is coupled to a hollow bottom sub 29. A bull plug 30 is coupled tothe bottom sub 29.

Still referring to FIG. 1 the coil assembly 25 has a housing 36, a core34, a first transmitting coil 31, a second transmitting coil 32,receiving coils 33, and separators 35. Separators 35, core 34, andhousing 36 may be constructed of a non-magnetic material.

The autonomous tool 10 is a casing collar locator. It can be loweredinto a well using a wireline. The example embodiment shown may alsooperate autonomously without a wireline. An advantage to not using awireline includes less surface equipment, fewer failure modes, andreduced cost. The autonomous tool can locate casing collars and countthose casing collars to determine how far down the borehole the tool hastraveled and determine its velocity through the borehole. The disclosedexample embodiment uses six coils to detect casing collars. Anothervariation of the tool may include using four coils instead of six coils.As the autonomous tool 10 travels through a wellbore it passes a casingcollar approximately every 40 feet of travel. The change in measuredelectromagnetic fields as the tool travels past a casing collarindicates that the tool is at the collar joint. By using a plurality ofcoils to detect collars the autonomous tool can measure collars withenough sensitivity to derive its velocity more accurately. Bycontinuously deriving velocity through the borehole, the autonomous tool10 can identify its position more accurately within a plus or minus tenfoot length zone even though the distance between each collar is fortyfeet in length. Furthermore, there may be other borehole magneticanomalies other than casing collars that the autonomous tool 10 may useto fine tune its calculations.

A variation of the autonomous tool 10 is to couple it to a frangibleperforating gun. The tool 10 may be dropped into a well and pump down.The autonomous tool 10 could detect casing collars and other parametersto accurately determine its position. Once the autonomous tool 10reaches its desired location it may then fire the perforating gun. Thegun and autonomous tool 10 would then be destroyed in the process ofperforating the well at a desired location. This variation of theautonomous tool 10 may function in a fire and forget perforating gunsystem.

A variation of the autonomous tool 10 may include the first housing 11and the second housing 12 are composed of a frangible material mayshatter when a proximately located perforating gun is fired. An examplemay include the autonomous tool 10 containing many components made ofceramic materials, including the first housing 11, the second housing12, the chassis 19, the PCB support 22, the top sub 16, the central sub24, the bottom sub 29, or the core 34. As an example the autonomous tool10 is deployed in a wellbore having casing 38 and casing 40 joined bycasing collar 39. A typical casing segment is 40 feet and each segmentis typically joined by a collar.

An assembly view of the autonomous tool is shown in FIG. 2 . Theautonomous tool 10 has a first housing 11, a second housing 12, a firstcentralizer 13, and a second centralizer 14. The first housing 11 has afirst end 37 and a second end 26. The first end 15 of the autonomoustool 10 ends with a top sub 16. The top sub 16 has a hollow center 17with a wire port plug 18. The first housing 11 is hollow and contains achassis 19 for holding a battery 20, data logger 21, and a printedcircuit board (PCB) support 22. The PCB support contains a PCB 23. PCB23 may include a processor or a plurality of processors and associatedelectronics. The processor may have memory for storing information, suchas programming, logging information concerning borehole magneticanomalies, or data recorded by the autonomous tool 10. A central sub 24is coupled to the second end 26 of the first housing 11 and the firstend 27 of the second housing 12. The second centralizer 14 is positionedover the first housing 11, second housing 12, and central sub 24. Thecentral sub 24 is hollow to allow for electronic connections to connectcomponents in the first housing 11 with components in the second housing12. The second housing 12 contains a coil assembly 25. The second end 28of the second housing 12 is coupled to a hollow bottom sub 29. A bullplug 30 is coupled to the bottom sub 29.

A close up of an example embodiment of the magnetic anomaly sensor 25 isshown in FIG. 3 . The coil assembly 25 has a housing 36, a core 34, afirst transmitting coil 31, a second transmitting coil 32, receivingcoils 33, and separators 35. Separators 35, core 34, and housing 36 maybe constructed of a non-magnetic material, such as a ceramic. There areno magnets in this design, thus allowing it to be used in a disposabletool that can be destroyed or left in the wellbore.

An assembly view of the autonomous tool combined with a perforating gunis shown in FIG. 4 . The autonomous tool 110 has a first housing 111, asecond housing 112, a first centralizer 113, and a second centralizer114. The first housing 111 has a first end 137 and a second end 126. Thefirst end 115 of the autonomous tool 110 ends with a top sub 116. Thetop sub 116 has a hollow center 117 with a wire port plug 118. The firsthousing 111 is hollow and contains a chassis 119 for holding a batteryand a data logger. It also contains a printed circuit board (PCB)support 122. The PCB support contains a PCB 123. A central sub 124 iscoupled to the second end 126 of the first housing 111 and the first end127 of the second housing 112. The second centralizer 114 is positionedover the first housing 111, second housing 112, and central sub 124. Thecentral sub 124 is hollow to allow for electronic connections to connectcomponents in the first housing 111 with components in the secondhousing 112. The second housing 112 contains a sensor 125. The secondend 128 of the second housing 112 is coupled to a hollow bottom sub 129.A perforating gun 150 is coupled to the bottom sub 129 using connectingsub 155. The perforating gun 150 has a housing 151, a charge holder 154,and shaped charge holes 152. Shaped charges are placed in the shapedcharge holes 152. In this configuration the autonomous tool may bedropped or pumped downhole. The perforating gun 150 has a bull plug 153.The sensor 125 may detect borehole magnetic anomalies and use thatinformation to determine its location in a wellbore. The autonomous toolmay have a second sensor located in the first housing 111. It may thencompare the measurements between the two sensors and compute itsvelocity by knowing the distance between the two sensors. It may alsocompare the measurements to data stored in the data logger to reconcileany discrepancies. By integrating the velocity over time the data loggeror other processors located within the autonomous tool 110 may determinethe distance it has traveled.

An alternative embodiment of the autonomous tool is depicted in FIG. 5 .In this example the autonomous tool 110 coupled to a perforating gun150, but in this example it is suspended by a wireline via cablehead 160with a housing 161 attached to the autonomous tool 110 using connectingsub 180.

An assembly view of an example embodiment includes an autonomous tool110 with two magnetic anomaly sensors combined with a perforating gun150 is shown in FIG. 6 . In this configuration there is a first sensor225 located in the first housing 112, or the top housing, and a secondsensor 125 located in the second housing 112, or the bottom housing.Sensors 225 and 125 may be magnetic anomaly detectors. In thisconfiguration there is a fixed distance between the first sensor 225 andthe second sensor 125. The autonomous tool 110 may include amicroprocessor and other auxiliary electronics on the PCB 123 that cancompare the measurements made by the first sensor 225 and the secondsensor 112. The autonomous tool 110 may then compute its velocity byknowing the distance between the two sensors and measuring the timedifference between the measurements made by the first sensor 225 and thesecond sensor 112. The processor may also compare the measurements todata stored on board to reconcile any discrepancies. By integrating thevelocity over time the data logger or other processors located withinthe autonomous tool 110 may determine the distance it has traveled. Theprocessor may calculate distance traveled by integrating the calculatedvelocity with respect to time. The processor may calculate the distancetraveled using a summation of the calculated velocity with respect totime. The processor may calculate the distance traveled by averaging thecalculated velocity over a plurality of measurements and multiplying bytime. The processor may calculate the distance traveled using apiecewise summation with respect to time. The processor may calculate orestimate distance traveled by comparing magnetic anomaly data receivedby first sensor 225 and second sensor 125.

An assembly view of an example embodiment includes an autonomous tool110 with a perforating gun 150 installed at its first end is shown inFIG. 7 . This is another configuration that may be desired depending onthe job at hand. Several different tools can be attached to theautonomous tool including perforating guns, setting plugs, jet cutters,braking tools, liner hangers, or other completions tools used in awellbore. Here the autonomous tool 110 has a bull plug 253.

An example embodiment may include the autonomous tool 110 as shown inFIG. 8 with a setting tool 300 coupled on the bottom sub 129 and a jetcutter 310 coupled to the top sub 116. A brake assembly may also beattached at a desirable location, such as in between the autonomous tool110 and the jet cutter 310. In this configuration the autonomous tool110 is dropped downhole or pump downhole. The sensor 125 and sensor 225provide the information needed for the on board electronics to determineits location in the wellbore. Once the autonomous tool 110 determines ithas reached a desired location it will deploy a brake. Then it willengage the setting tool 300 via connecting sub 303, causing seal 301 toengage the wellbore. Seal 301 may be a metal seal, a packer, a rubberseal, or a combination of various materials in a multitude of shapes forthe purpose of sealing a wellbore. The setting tool 300 has a piston304, a rod 305 that collapses the seal 301 by compressing it withhousing 302, causing seal 301 to bulge out against a casing wall. Nextthe autonomous tool 110 will activate the jet cutter 310, which willsever the downhole tubulars, including the casing with explosives. Theautonomous tool is then abandoned in place. The casing that was severedmay then be pulled from the wellbore if necessary. In this operation theautonomous tool 110 works as a disposable plug and abandonment tool.

One of the purposes of the disclosed embodiments is to accuratelyidentify casing collars as the tool is either freefalling or beingpumped down a cased hole. Pumping the tool downhole may be necessary forhorizontal wells. One issue is that there are other anomalies that mayconfuse a more traditional casing collar locator. The use of twodifferential spaced magnetic sensors and digital signal processingmatching algorithms may continuously determine the velocity of the tool.The tool may then calculate the distance the tool has traveled. Thedistance calculation may include integrating the velocity over time,summation of the discrete velocity data, average the velocityinformation multiplied by time, or a piecewise summation method. Thetool may start measuring velocity as soon as it enters the wellbore. Thetool may use collars, anomalies, and/or both to determine velocity. Bydetermining the distance traveled accurately, the tool can performcertain functions at a pre-determined location in the well includingsetting a plug, cutting pipe, or detonating a perforating gun.

Although the invention has been described in terms of particularembodiments which are set forth in detail, it should be understood thatthis is by illustration only and that the invention is not necessarilylimited thereto. For example, terms such as upper and lower can besubstituted with uphole and downhole, respectfully. Top and bottom couldbe left and right. The first housing and second housing may be tophousing and bottom housing, respectfully. Terms like wellbore, borehole,well, bore, oil well, and other alternatives may be used synonymously.The alternative embodiments and operating techniques will becomeapparent to those of ordinary skill in the art in view of the presentdisclosure. Accordingly, modifications of the invention are contemplatedwhich may be made without departing from the spirit of the claimedinvention.

What is claimed is:
 1. An apparatus for use downhole comprising: a tophousing with a first end, a second end, an axis, and a first centralizersurrounding a portion of the first end; a bottom housing with a firstend located proximate to the second end of the top housing, and a secondend, wherein the bottom housing is coaxial with the axis, and a secondcentralizer surrounding a portion of the second end of the top housingand a portion of the second end of the bottom housing; a first magneticanomaly sensor located within the top housing further comprising aplurality of electromagnetic coils wherein a first and secondelectromagnetic coil generates an electromagnetic field and a third,fourth, fifth, and sixth electromagnetic coils detect electromagneticfields; a second magnetic anomaly sensor further comprising a pluralityof electromagnetic coils wherein a seventh and eighth coil generates anelectromagnetic field and a ninth, tenth, eleventh, and twelfth coilsdetect electromagnetic field, located within the bottom housing andlocated a fixed axial distance from the first magnetic anomaly sensor;and a processor located within the top housing, operatively connected tothe first magnetic anomaly sensor and the second magnetic anomalysensor, wherein the processor calculates the velocity of the apparatusbased on comparing measurements taken from the first magnetic anomalysensor and the second magnetic anomaly sensor.
 2. The apparatus of claim1 further comprising a first sub coupled to the first end of the tophousing.
 3. The apparatus of claim 2 further comprising a second subcoupled to the second end of the top housing and coupled to the firstend of the bottom housing.
 4. The apparatus of claim 3 furthercomprising a third sub coupled to the second end of the bottom housing.5. The apparatus of claim 1 wherein the first centralizer is a hollowcylindrical shape.
 6. The apparatus of claim 5 wherein the secondcentralizer is a substantially hollow cylindrical shape.
 7. Theapparatus of claim 1 further comprising a cylindrical core locatedcoaxial with the axis and passing through the first, second, third,fourth, fifth, and sixth electromagnetic coils.
 8. The apparatus ofclaim 1 further comprising a cylindrical core located coaxial with theaxis and passing through the first, second, third, fourth, fifth, andsixth electromagnetic coils.
 9. The apparatus of claim 1 wherein theprocessor includes a data logger.
 10. The apparatus of claim 1 whereinthe processor includes a plurality of processors.
 11. The apparatus ofclaim 1 wherein the processor computes the velocity by comparingmeasurements taken from the first magnetic anomaly sensor and the secondmagnetic anomaly sensor.
 12. The apparatus of claim 1 wherein the tophousing is composed of a frangible material.
 13. The apparatus of claim1 wherein the top housing is composed of a ceramic material.
 14. Theapparatus of claim 1 wherein the bottom housing is composed of afrangible material.
 15. The apparatus of claim 1 wherein the bottomhousing is composed of a ceramic material.
 16. The apparatus of claim 1wherein the processor calculates distance traveled by integrating thecalculated velocity with respect to time.
 17. The apparatus of claim 1wherein the processor calculates the distance traveled using a summationof the calculated velocity with respect to time.
 18. The apparatus ofclaim 1 wherein the processor calculates the distance traveled byaveraging the calculated velocity over a plurality of measurements andmultiplying by time.
 19. The apparatus of claim 1 wherein the processorcalculates the distance traveled using a piecewise summation withrespect to time.